1. Field of the Invention
The present invention relates to a discharge tube, and more particularly to a discharge tube having a stabilized discharge inception characteristics suitably used for a voltage controller, a gap switch, a sharpener gap and so on.
2. Description of the Related Art
Generally, a discharge tube is adopted in a voltage controlling device, or in a gap switch for correctly controlling a discharge inception of a pulse laser or the like, and as a discharge tube suitable for this purpose there has been provided a discharge tube which comprises a cylindrical container made of an insulation tube provided with a pair of discharge electrodes at opposite sides thereof, and having an inert gas sealed therein.
The discharge tube as constructed above in general has a tendency that when the frequency of repetitive discharge becomes low, a discharge inception is delayed and a discharge inception voltage is thereby raised. However, it is not desirable that the discharge inception voltage varies according to the discharge frequency, and in fact, a discharge inception voltage in a discharge tube should be stabilized regardless of the above frequency of repetitive discharge.
In view of these defects to be overcome, there has been invented a discharge tube which is capable of accelerating an inception of the main discharge by providing trigger lines made of conductive layers or the like respectively extending from opposite electrodes on the inner surface of a cylindrical container which is a main body of the discharge tube. However, in the discharge tube as constructed above, when the discharge is repeated for a long time, the trigger lines are worn and thereby the trigger effect thereof does not last long, and in addition, an abnormal discharge creeping, that is, so called an inner creeping discharge along the inner surface of the cylindrical container is likely to occur.
In view of the above phenomena, there has also been proposed a discharge tube which is capable of accelerating a preionization of the gas sealed in a discharge space thereof, avoiding thereby the delay of an inception of discharge even when the frequency of repetitive discharge is substantially low by dispersively building up a small amount of electrically conductive substance on the inner surface of the cylindrical container (as shown in Japanese Utility Model Application Laid-open No. 3-68389), or, for example as shown in FIG. 8A, by coating an insulation coating substance containing a gas-ionization accelerator such as a silicate compound of alkaline metal on the inner surface of the cylinder (as shown in Japanese Patent Application Laid-open No. 4-133244). However, even a discharge tube as constructed above could not perfectly solve such a problem that the inner creeping discharge is more likely to occur in proportion to the cumulative number of discharge, in spite of the fact that it is provided with a stabilized discharge inception.
Under these circumstances, the applicants of the present invention have found out the fact that a discharge tube as shown in FIG. 8B, which is constructed such that a plurality of insulation coated lines 3 containing a gas-ionization accelerator therein are provided on the inner surface of a cylindrical container 1 in such a way that they cover the regions surrounding a cathode 4 and a discharge space formed between the cathode 4 and anode 5 in a substantially parallel relation with a center axis of the cylindrical container 1, has a stabilized discharge inception characteristics regardless of the frequency of repetitive discharge or of the cumulative number of discharge, yet capable of eliminating an occurrence of inner creeping discharge, and accordingly the applicants have filed a Japanese Utility Model Application (Laid-open No. 4-69539). However, even though a discharge tube as constructed above could suppress an inner creeping discharge better than the conventional ones, it has still been a difficult problem to control the length and width of the insulation coated lines 3 of each electrode into a constantly same size to provide a stabilized life time to all the tubes.
The present invention has been made to eliminate such problems, and it is an object of the present invention to provide a discharge tube which has a stabilized discharge inception characteristics regardless of the frequency of repetitive discharge or of the cumulative number of discharge, and is also capable of eliminating an occurrence of inner creeping discharge, yet having a constant life time.